The use of hematopoietic stem cells and their progeny through bone marrow transplants to reconstitute the hematopoietic system has been employed to treat various blood-related diseases and disorders such as aplastic anemia, immune deficiencies and several forms of cancer including lymphomas and leukemias (see review in Lu et al. Critical Rev. Oncol/Hematol. 22:61–78 (1996)). Bone marrow transplantation is most commonly used in an attempt to restore hematopoietic function following exposure to myeloablative agents, for example after radiation therapy or chemotherapy in the treatment of a variety of cancers. These therapies, in addition to destroying the cancer, can also result in myelosuppression or myeloablation which, in turn, can lead to infection, bleeding disorders, and other complications. Recent estimates suggest that the need for transplantation of bone marrow-derived hematopoietic stem cells is growing at a rate of 20% per year and the market for the product is approximately $500 million per year (Strickland, D. Bioworld Today 8(14):1).
Unfortunately for patients, the use of bone marrow transplantation as a therapy is very restricted. Several disadvantages are inherent to the use of hematopoietic cells as a source of cells in the treatment of blood-related disorders and diseases. The success of an allogeneic transplant usually depends on finding a donor who is histocompatible with the recipient and willing to be subjected to the painful and time-consuming bone marrow donation process. Genetic incompatibility can lead to two common and potentially lethal complications. First, to decrease the chances of host versus graft reaction, the patient's immune system is compromised through the use of immunosuppressive drugs, leaving the patient highly susceptible to infection. Second, once the transplanted marrow cells are established they sometimes attack the patient in a graft versus host reaction. Combined, these two factors account for the major causes of non-autologous bone marrow transplant patient mortality and morbidity.
As an option to allogeneic transplantation, a patient's own bone marrow can sometimes be harvested and stored for later use assuming that the patient is healthy enough to withstand the procedure, and that the marrow is useful. Although the employment of such an autologous system generally precludes the danger of a genetic mismatch, serious risks still exist from possible undetectable contamination with malignant cells. The reliable detection and elimination of transformed marrow cells has yet to be accomplished. A further disadvantage with this approach is that only a limited amount of bone marrow cells capable of completely reconstituting the hematopoietic system can be harvested from an individual.
There has been much effort in establishing ex vivo culture systems for hematopoietic stem and progenitor cells for the purpose of generating a sufficient number of cells for transplantation purposes. However, the procurement of sufficient quantities of hematopoietic stem cells, either through bone marrow biopsy or from other sources, is a limitation to the use of this tissue for hematopoietic system related therapies. Present systems require complex culture conditions and tedious cell separation steps, and result in only a limited expansion of the numbers of hematopoietic stem cells. See, for example, U.S. Pat. No. 5,646,043, to Emerson et al. The biggest drawback is the lack of ability to sequentially passage the stem cells in vitro under defined culture conditions, over an extended period of time, in order to expand the numbers of functional cells available for transplantation. (Amos & Gordon, supra; Lu, et al., supra). As a consequence, there exists an ongoing need to either repeatedly harvest autologous stem cells or recruit compatible donors for therapies involving reconstitution of the hematopoietic system.
Due to the complications mentioned above, other sources of stem cells for hematopoietic reconstitution have been sought. Studies on the employment of fetal liver cells, neonatal spleen cells, or thymus cells have been reported. (Amos & Gordon, supra; Lu et al., supra). However, the ethical issues related to employing these cell types make the commercial use of them less attractive. The possibility of harvesting and cryopreserving cord blood is currently being studied, and may provide a more acceptable means of procuring cells for future use (Broxmeyer et al., (1989) Proc. Nat. Acad. Sci. USA 86:3828). To date, however, cord blood derived cells have only been shown capable of successfully repopulating the hematopoietic system of children (Amos & Gordon, supra). The recent identification of peripheral blood progenitor cells (PBPC) with marrow repopulating abilities has opened investigations into the use of PBPC for transplantation (reviewed in Lu et al., supra). Harvesting of these cells would potentially replace the need for bone marrow transplants. However, several disadvantages are apparent with this system: 1) the harvested blood may be contaminated, 2) the incidence of graft versus host disease is still very high, 3) many runs of leukapheresis are required to collect enough circulating stem cells for a complete hematopoietic reconstitution, and 4) collected stem cells cannot be held in an undifferentiated state for long periods of time.
It is apparent from the foregoing that alternatives are needed to present methods for reconstitution of a patient's hematopoietic system.